This invention relates to processing of oxide superconductor composites to obtain strong grain coupling and high critical current density oxide superconductor articles.
Polycrystalline, randomly oriented oxide superconductor materials are generally characterized by their low density, poor grain connection and low critical current densities. High oxide density, good oxide grain alignment and grain interconnectivity, however, are associated with superior superconducting properties.
Composites of superconducting materials and metals are often used to obtain better mechanical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by a well-known powder-in-tube or PIT method. When powders include metal oxides or other oxidized metal salts, the method is referred to as oxide-powder-in-tube or OPIT. For filamentary articles, the method generally includes the three stages of (a) forming a powder of superconducting precursor materials (precursor powder formation stage); (b) filling a noble metal billet with the precursor powder, longitudinally deforming and annealing it, forming a bundle of billets or of previously formed bundles for multifilamentary forms, and longitudinally deforming and annealing the bundle to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material surrounded by a noble metal matrix (composite forming stage); and (c) subjecting the composite to successive asymmetric deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermo-mechanical processing stage). General information about the OPIT method described above and processing of the oxide superconductors is provided by Sandhage et al. in JOM, 43(3):21-25 (1991), and references cited therein; by Tenbrink et al., “Development of Technical High-Tc Superconductor Wires and Tapes,” Paper MF-1, Applied Superconductivity Conference, Chicago (Aug. 23-28, 1992); and by Motowidlo et al., “Properties of BSCCO Multifilamentary Tape Conductors,” Materials Research Society Meeting, Apr. 12-15, 1993, all of which are incorporated by reference.
The deformations of the thermo-mechanical processing state are asymmetric deformations, such as rolling and pressing, which create alignment of precursor grains in the core (textured grains) and facilitate the growth of well-aligned and sintered grains of the desired oxide superconducting material during the later thermal processing stages. A series of heat treatments is typically performed during the thermo-mechanical processing stage to convert the partially reacted oxide powder to Bi2Sr2Ca2Cu3Ox(BSCCO-2223 or 2223) superconducting phase, where x is selected to provide superconductivity at temperatures above 77K, in order to crystallize the desired highly textured superconducting layer and to obtain strongly coupled superconducting grains.
In the practice of the above prior art approach, it has been found that in the final thermo-mechanical processing stage, some low Tc superconducting phases, such as BSCCO-2212 (2212) and BSCCO-2201 (2201), and non-superconducting phases, such as Ca2PbO4 and (B,P)SCCO-3221 (3221), may remain or reform during the cooling stage, as observed by Poulsen et al. using a synchrotron X-ray diffraction method (Physica C 298:265 (1998)). These phases may stay at grain boundary and reduce grain coupling, which is one of the most effective factors limiting superconducting transport property. See, Kaneko et al. Advances in Superconductivity IX. Proceeding of the 9th International Symposium on Superconductivity: 907 (1997).
Current approaches to improving grain connectivity include treating the material at reduced temperatures after liquid-phase sintering. For example, Li et al. in U.S. Pat. No. 5,798,318, report processing a BSCCO material by liquid-phase sintering at 820-840° C., immediately followed by heat treatment at 750-830° C. to fully convert the liquid phase and remaining 2212 and 2201 phases into 2223 phase and then at 600-750° C. to reduce lead-rich phases, such as Ca2PbO4 and 3221, at the grain boundaries. Liu et al. (Physica C 325:70 (1999)) reported that the remaining 2212 and liquid phases convert to 2223 phase and that superconducting transport performance is significantly improved by sintering at 840° C. followed by annealing at about 825° C. Zeimetz et al. (Superconductor Science and Technology 11:505 (1998)) disclose a heat treatment at 820° C. (50 h) after 2223 phase formation, which reduces the 2201 phase at the grain boundary. Recently, Hua et al. (Physica C 339:181 (2000)) also reported that annealing at 780° C. after a phase converting heat treatment (herein after “post-annealing”) increases the Tc of the remaining 2212 phase and improves transport property. Another example of post-annealing to reduce the amount of lead-rich and 2201 phases includes cooling to room temperature between liquid phase sintering at 839° C. (50 h) and post-annealing at 819° C. (100 h) (Lehndorff et al., Physica C 312:105 (1999)). This treatment significantly reduces 2201 and 3221 phase, and increases 2223 and 2212 phases as well.
These approaches represent attempts to improve grain connectivity by baking or annealing at reduced temperature after liquid phase sintering. The remaining liquid phase and 2212 and 2201 phases, as well as lead-rich phases such as Ca2PbO4 and 3221 phases that are formed during the cooling stage, are removed to some extent by this type of post-annealing.
A different approach to improving oxide phase formation and oxide grain connectivity is to control the cooling rate after liquid phase sintering. Many groups have studied the cooling rate effect on transport property; however, no consistent results are reported because the starting materials for these studies had different phase compositions. Huang et al. (Superconductor Science and Technology 7:795 (1994)) identifies 50° C./h as an optimal cooling rate because fast cooling causes less 2223 phase decomposition. Singh et al. (Journal of Materials Research 13:261 (1998)) report that cooling at 5° C./h produces higher phase purity in some tapes above 700 C., but that below 700° C. slow cooling leads to 2223 phase instability.
Most experiments described above were carried out on short tapes, which are not always predictive of performance in long lengths of material. For example, oxygen diffusion through matrix material is not as critical an issue in short lengths as it is in long lengths of tape. Because lead-rich phase formation relates to oxygen absorption, slower oxygen diffusion rate through the matrix in long tapes delays a lead-rich phase formation, and the lead-rich phase remains at the grain boundary when forming at lower temperature.
Thus, there remains a need to further improve grain connectivity by increasing phase purity and reducing lead-rich phases as well as other secondary phases at grain boundary, especially for long length material.